I met a guy in the street from Fort Worth in Texas, called Peter.
Then on another occasion I met another guy from San Antonio in Texas, called Jack.
I said to Jack, the second guy, 'I know a guy from Fort Worth.'
He said, 'Not Peter? He's a student.'
I said, 'Yes, he's doing a PhD.'
I guess the guys from Texas hang together.
I haven’t seen Peter since that first occasion, but if I do see him again, I’ll say to him,
"I met Jack, the guy from San Antonio"
My question is why “the”? There are many guys from San Antonio, and I don’t know, perhaps Peter knows other guys from San Antonio besides Jack.
I’m not going to say,
"I met Jack, a guy from San Antonio."
Peter knows Jack. There are many guys from San Antonio, and perhaps Peter knows other guys from San Antonio besides Jack.
But still, Jack, the guy from San Antonio.
I think it’s because the function of ‘the’ is to tell the listener that they can identify the referent from the information we have shared. I would be telling Peter that he knows who I am referring to when I say, ‘the guy from San Anonio’, because I shared the name ‘Jack.’
Of course he could come back and say, ‘I know 2 guys called Jack from San Antonio’. Or ‘I don’t know anyone called Jack.’
Then I could describe the Jack I met.